Download duality, IMHO it's the safest and best way to change themes in OS X.
<a href="http://conundrumsoft.com/Duality/duality.html" target="_blank">http://conundrumsoft.com/Duality/duality.html</a>
Install duality and create a "Themes" folder in the main library of your system (it might do this for you), drop some <a href="http://www.interfacestudio.net/Themes/" target="_blank">.themes</a> in it and open the app to change themes.